A pass is complete when there is no overlap between the two cars.
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As a corner worker I was tought that if two cars are approaching the line at the flag station while overlaped and the rear most car is increasing the overlap he or she is defacto passing. The person must either complete the pass or back off before crossing the line other wise he or she is guilty of PUY. [/b]
A pass occurs when the nose of the overtaking car breaks the plane of the the overtaken car. If the overtaking car breaks that plane, it does not matter whether there is overlap - a pass has occured. This is the standard that would be used if the cars were being scored at the S/F line.

A following car may decrease any overlap without committing a pass under yellow - as long as the plane is not crossed. Pass - verb to move past; go by: to pass another car on the road. There is nothing in the GCR that says that I cannot gain an advantage (i.e. reduce the overlap) under a yellow flag.

Now, the overtaking car might be guilty of not displaying sufficient caution, but that's another issue and much more difficult to prove since it is a judgment call.